There are many techniques for developing latent fingerprints on various substrates know in the world of forensic science. Fingerprints consist of approximately 98% water with the remaining 2% being a combination of grease, oil, salts and amino acids. The ability to develop these fingerprints varies with the type of substrate on which they are deposited. Porous substrates such as thermal paper require chemical treatments to develop latent prints, the most commonly used chemical techniques including ninhydrin(triketohydrindene hydrate) spraying, 1,8-diazafluoren-9-one (DFO) treatment, and silver nitrate and iodine fuming. Development of latent prints with ninhydrin depends on a chemical reaction with the amino acids left by the fingerprint to form a purple-blue colored product, Ruhelmann's Purple. The ninhydrin spray is prepared using ninhydrin powder and a suitable solvent, such as acetone or ethanol, or from ninhydrin crystals and a solvent. The use of DFO follows a similar reaction path, wherein amino acids react with the DFO to form a product that fluoresces yellow under green excitation light. There are several disadvantages associated with these conventional methods. In particular, the development of latent prints on thermal paper can take as long as three weeks using conventional ninhydrin methods. Further, the thermal paper can be discolored if the chemicals are not prepared or applied properly, which can result in damage or loss of valuable evidence.
Cyanoacrylate (CA) esters have been used since the late 1970s as an effective means of developing latent fingerprints on non-porous substrates such as glass, plastics and aluminum foil. CA fuming utilizes the CA esters to develop the latent prints by heating liquid CA (e.g. super glue methylcyanoacrylate or ethylcyanoacrylate) so that it reacts with traces of amino acids, fatty acids and proteins in the latent fingerprint and moisture in the air to produce a visible, sticky white material that forms along the ridges of the fingerprint. The final result is an image of the entire latent fingerprint that may be photographed directly but often requires a separate process for further enhancement. Such further enhancement usually comprises treating the article with a premixed liquid dye solution, applied by dipping, spraying, or brushing followed by drying. The disadvantages of using further enhancement processes includes the fact that enhancement dyes are usually mixed by the facility trying to develop the latent prints, which can easily result in inconsistency or errors in formulation and concentration. This can in turn lead to damage or loss of the evidence. In addition, the dye solutions contain solvents that may dissolve the fingerprint residue that has not reacted with the CA. Furthermore, the action of dipping, spraying or brushing the evidence with the dye solution may wash away or wipe away critical evidence.
While the CA procedure represents the most common method for the development of latent fingerprints, it is relatively complex and time consuming. In particular, to enable the reaction to take place, the CA must be in gaseous form, which is generally accomplished by placing the article into a fuming chamber or an airtight tank having a small heater. A few drops of liquid CA are then placed into a tiny, open container and the container is placed on top of the heater inside the tank. The tank is then carefully sealed but remains at atmospheric pressure and the heater is activated. The boiling point for most CAs varies between 49° C. and 65° C. (120° F. and 150° F.) depending on the chemical composition. When the CA in the container reaches the boiling point it will boil away into the surrounding atmosphere to create a concentration of gaseous CA. If any latent fingerprints exist anywhere inside the tank, they will eventually be exposed to the gaseous CA and the natural humidity contained in the atmosphere is enough to trigger the reaction. The whole reaction can take over two hours depending on the size of the tank, the concentration of the gaseous CA in the air, the humidity in the air and numerous other factors. There are a number of disadvantages associated with using CA development processes, including overdevelopment when polymerization occurs between the fingerprint ridges and excess development beyond the ridges that make it difficult to distinguish the print from the background. In addition, CA polymerizes to form a white material that may not provide contrast against a white background, again making distinguishing of the print difficult. This often results in the need for further enhancement, e.g. the use of fluorescent dyes to visualize the prints.
Another method for developing latent fingerprints is through the use of Physical Vapor Deposition (PVD), the most common method being Vacuum Metal Deposition (VMD). When a human fingerprint touches a surface, a sweat deposit, i.e. the fingerprint is left behind on the surface. A standard VMD method uses a vacuum chamber and thermal sources to evaporate two thin layers, one of gold and the other of zinc, onto the suspect material. The material is loaded into a vacuum chamber and the pressure is lowered to approximately 1 millionth of an atmosphere (10−4 mbar). Gold is then evaporated, deposits uniformly over the surface and is absorbed by the ridges of the human sweat that form the invisible fingerprint pattern. Zinc is then evaporated and because zinc generally condenses only onto another metal, it adheres only to the gold coated areas that lie between the sweat ridges to create a visible pattern defining the latent fingerprint image. This method of developing fingerprints is again relatively complex and time consuming, as well as costly.
There is a need in the art for improvements in the development and enhancement of latent fingerprints.